This invention relates to magnetic field detection and in particular to optical fibre magnetometers for determining magnetic fields or magnetic field gradients.
Interferometric fibre optic magnetometers are already known for example from "A fibre-optic dc magnetometer" by K. P. Koo et al. Journal of Lightwave Technology, Vol. LT-1, No. 3, Sept. 1983, p 524-5. They basically comprise an all-fibre Mach-Zehnder interferometer, one arm of which is magnetically sensitised by, for example, being bonded to a strip of magnetostrictive material or coated with a magnetostrictive material. A d.c. magnetic field has the effect of altering the optical path length of the sensitised arm and the interferometer output which is proportional to the differential path length change (.DELTA.L) is thus related to the magnetic field (H), .DELTA.L .delta.H.sup.2. To overcome inaccuracies due to noise etc an a.c. bias field at frequency w is applied to the sensitised arm and then the interferometer output at frequency w is directly proportional to the d.c. magnetic field.
A fibre-optic magnetic gradient detector is described by K. P. Koo et al in the article on pages 509-513 of the publication referred to above. In this case both arms of the all-fibre Mach-Zehnder interferometer are magnetically sensitised and a.c. bias fields at the same frequency are applied to both sensitised portions. The detector described in this article only measures the magnetic gradient in one direction, that is between the two arms.
In our co-pending G.B. Application No. 8504729 (P. Extance-R. E. Jones 17-17) there is described a fibre optic magnetic gradient detector which detects two orthogonal magnetic gradient components simultaneously and employs three sensitised optical fibre portions, two in one arm and one in the other arm of an all-fibre Mach Zehnder interferometer, and a.c. bias fields at two different frequencies.